1. Field of the Invention
The present invention relates to a positive displacement pump which moves fluid by changing the volume of its pumping chamber with a piston or diaphragm, and, more particularly, it relates to a highly reliable pump with a high flow rate.
2. Description of the Related Art
Conventionally, typical pumps of this type have a check valve installed between an inlet flow path and a variable-volume pumping chamber as well as between an outlet flow path and the pumping chamber, as described, for example, in Japanese Patent Laid-Open No. 10-220357.
Also, there are pumps which produce unidirectional flow by utilizing viscous drag and are configured, for example, as described in Japanese Patent Laid-Open No. 08-312537 such that fluid resistance is larger in the inlet flow path than in the outlet flow path when a valve installed in the outlet flow path is open.
Furthermore, there are pumps which have compression components configured such that pressure drops vary with the flow direction both in inlet and outlet flow paths in order to improve the reliability of the pumps without using movable parts for valves, as described, for example, in National Publication of International Patent Application No. 08-506874 and in a paper “An improved valve-less pump fabricate using deep reactive ion etching” on pp. 479–484 of 1996 IEEE 9th International Workshop on Micro Electro Mechanical Systems.
However, the configuration described in Japanese Patent Laid-Open No. 10-220357 requires a check valve both in the inlet and outlet flow paths and has the problem that a fluid passing through two check valves suffers high pressure loss. Also, the check valves, which open and close repeatedly, are liable to fatigue damage. Besides, the larger the number of check valves, the lower the reliability.
Also, the configuration described in Japanese Patent Laid-Open No. 08-312537 needs to increase the fluid resistance in the inlet flow path in order to reduce back-flow in the inlet flow path during the discharge stroke of the pump. Consequently, the suction stroke of the pump, during which the fluid is introduced into the pumping chamber against the fluid resistance, becomes considerably longer than the discharge stroke. This results in a significantly low frequency of cycling between the pump's suction and discharge strokes. With a pump in which a piston or diaphragm moves up and down, generally the higher the frequency of the up-down movement, the higher the flow rate and power output, provided the area of the piston or diaphragm is constant. However, since the configuration described in Japanese Patent Laid-Open No.08-312537 allows only low-frequency operation as described above, it cannot implement a small, high-power pump.
Furthermore, in the case of the pump described in National Publication of International Patent Application No. 08-506874, since it is configured to produce unidirectional net flow of the fluid passing through the compression components as the volume of the pumping chamber increases and decreases, using the pressure drops which vary with the flow direction, the back-flow increases with increases in external pressure (load pressure) on the outlet side and the pump fails to operate under high load pressure. According to the paper“An improved valve-less pump fabricate using deep reactive ion etching,” the maximum load pressure is around 0.760 atmosphere.
The present invention has been made to solve the prior art problems described above. Its object is to provide a small, lightweight, high-power pump which can operate even under high load pressure.